An oil agent for a carbon fiber precursor containing an ester compound and a nitrogen-containing surfactant (see, for example, Patent Document 1); an oil agent for a carbon fiber precursor containing a modified silicone, a polybasic acid, and a salt derived therefrom (see, for example, Patent Document 2); an oil agent for a carbon fiber precursor containing a modified silicone and an alkaline compound (see, for example, Patent Document 3); an oil agent for a carbon fiber precursor containing an ester compound and a water-soluble amide compound (see, for example, Patent Document 4); and an oil agent for a carbon fiber precursor containing an amino-modified silicone, a nonionic surfactant, and an antistatic agent (see, for example, Patent Document 5) have been known conventionally. However, problems thereof are that antistatic properties and convergence properties that each of the conventional oil agents for a carbon fiber precursor imparts to a carbon fiber precursor are insufficient, and that a machine for producing a carbon fiber precursor rusts easily.